Large load-supporting structural surfaces, either vertical, horizontal or a combination of both, are in universal and widespread use. These structures must support their own weight and, normally, very large loads such as layers of ground and soil of as much as 30 to 40 or more feet high, heavy payloads such as bridge traffic and the like. Since these structures are necessarily large, that is since they have long, essentially unsupported spans of as much as 50 to 100 feet in length and more they are subjected to very large forces and deflections which could in the past only be handled with elaborate fabricated support beams and trusses, with massive reinforced concrete walls and beams, or with a combination of both.
Fabricated steel structures, though not excessively heavy, are expensive because they use a relatively large amount of expensive material, e.g., high quality steel which must be tediously fabricated, assembled and installed from a multiplicity of different, individually fabricated members such as I-beams, angle irons, plates and the like welded, riveted or bolted together. Furthermore, to obtain the necessary strength such structures required a great depth, often of many feet, which might not be available, or which is only available at significant costs, e.g., by performing expensive excavation and the like.
As an alternative to such fabricated metal structures, reinforced concrete has found increasing acceptance. Frequently the concrete structures are aesthetically more appealing and they are often less expensive. Nevertheless, they require the erection of complicated forms and the installation of the necessary reinforcing steel bars all of which requires individual, on-the-site fabrication, assembly and installation by skilled and, therefore, costly craftsmen.
After the necessary large volume of concrete has been poured into the forms and the forms have been dismantled the concrete structures are again quite expensive. Moreover, they too have to be massive to support a given load.
To overcome some of these shortcomings and to reduce construction costs, it has in the past been suggested to employ prefabricated plate, normally steel plate elements. Since plate as such is weak, that is since it cannot withstand large forces acting perpendicular to the plate, it has also been suggested to employ corrugated plate structures. Examples of such constructions are disclosed, for example, in U.S. Pat. Nos. 2,126,091; 2,536,759; 3,508,406; and 3,638,434.
The referenced patents disclose tunnel-like, load-supporting structures made of corrugated plate, that is relatively short sections of corrugated plate normally having corrugations with a pitch of up to 6 inches, a corrugation depth up to 2 inches, and a wall thickness of up to three-eighth inch. For the contemplated large structures, which have a width (perpendicular to the tunnel defined by the structure) of up to 60 feet and more, it is necessary to include stiffening members which rigidify the structure both for load-bearing purposes and for maintaining the structure in the desired, e.g., normally arched shape during the backfilling and compacting process. Even then such structures exhibit relatively little load, e.g., ground supporting capacity unless the structure is reinforced with suitable stiffeners and the like. As a consequence, these structures, though relatively less expensive because they could be assembled from uniform, prefabricated modules, i.e. like, prefabricated and, where applicable, curved corrugated plate elements, their relatively low strength limited their application to relatively short span lengths and relatively small loads. For example, typical highway overpasses which have to accommodate a ground fill height of 10 to 30 and more feet as well as a large payload such as a standard California State Highway surcharge of H20 (for standard freeway traffic) must be built as before from fabricated steel and/or reinforced concrete both of which renders such structures relatively expensive.
In other instances in which relatively long, loadbearing spans are required, such as in large bulk material, e.g., gravel storage bins, bin type retaining walls were suspended between upright posts and constructed of multiple, prefabricated, U-shaped members made from steel plate of the appropriate thickness which was press-formed to the desired shape. By providing the resulting U-shaped channel members with the appropriate depth the required strength could be obtained. The inherent shortcoming of this approach is that the maximum span length is limited by the effective length of the longest available press. Moreover, such fabrication method is tedious, each channel member must be separately fabricated and thereafter the channel members must be assembled, usually bolted together in a side-by-side relationship to form a wall of the desired height. The resulting structure, though having adequate strength but not necessarily an adequate length, was relatively expensive.
Thus, the prior art applicable to structures here under consideration, that is structures having relatively large load-bearing surfaces that are unsupported between ends of the surfaces such as are found in bridge, tunnel or retaining wall constructions, can be summarized as relying on fabricated steel or reinforced concrete or a combination of both to attain the necessary strength and stiffness. Both of these approaches require a great deal of hand labor and material, and therefore, time to assemble and install, all of which renders them relatively expensive. It has been recognized that prefabricated, modular metal plates are relatively less expensive to produce, assemble and install, however, these plates exhibited severe strength limitations and could only be used for relatively small structures unless suitable stiffeners and supports were provided and unless the structure under consideration had the necessary shape to not only be self-supporting but to also support a payload. This latter aspect required that the structures be tubular and continuously arcuate as distinguished from U-shaped, or tubular with straight walls of the like even if the latter shape is more desirable for the structure under consideration.